1. Field of the Invention
This invention relates to active vibration control systems particularly but not exclusively for use in reducing vibration of a helicopter fuselage.
2. Description of the Prior Art
In active vibration control systems one or more controlled excitations are applied to a vibrating structure to alleviate a vibration response generated by one or more uncontrolled excitation sources. Typically, the sources of the controlled excitations are actuators controlled so as to produce a vibratory force or motion and, in the helicopter case, the source of uncontrolled vibration is a main sustaining rotor. Unlike alternative passive techniques in which a device responds mechanically through feedback of vibration local to the device, active systems are based on the feedback of vibration, usually from sensors dispersed around the structure or airframe, so as to adapt the controlled excitations to account for changes in the uncontrolled vibration forcing which, in the helicopter case, will arise due to changes in operating conditions.
Several techniques based on differing control philosophies have been proposed for implementing active vibration control for alleviating vibration of a helicopter fuselage. One such technique known as Higher Harmonic Control (HHC) is based on the application of vibratory pitch motions to the rotor blades of a main sustaining rotor to reduce the primary vibratory forcing at its source. Another technique has attempted to actively isolate a structure such as a helicopter fuselage from the primary vibration source. Both these techniques suffer performance constraints, the former in terms of power requirements and high speed performance and the latter in terms of a high weight requirement for effective isolation of all the vibratory load paths.
U.S. Pat. No. 4,819,182 describes an active vibration control system which overcomes the problems associated with such prior techniques. Termed "Active Control of Structural Response" (ACSR), the system differs from prior techniques in that it is based on the superposition of the uncontrolled vibration response and the controlled vibration response, which is controlled in such a way that vibration is minimised throughout the structure.
Thus, whilst ACSR uses the basic system components of actuators, controller and sensors of the prior active techniques described above, it operates in a fundamentally different way. ACSR differs from HHC in that the controlled excitations or forces are applied by the actuators at or across points of the structure capable of relative motions in the dominant vibratory modes. The actuators can either be connected between two points of the structure, e.g. between a gearbox and a fuselage, in what is termed a dual point actuation system or alternatively can be connected between the structure at one point and a seismic mass to generate the required forcing. The latter arrangement is termed a single point actuation system. ACSR differs from prior isolation techniques since it makes no attempt to control directly the vibratory load paths across an isolation interface.
ACSR incorporating frequency domain control logic has proved highly successful in helicopters by reducing overall vibration of the fuselage by a value of about 80 percent and localised improvements of over 90 percent. However, transient delays in each control cycle may impose limitations preventing further improvement in such systems. Similarly, ACSR systems incorporating alternative time domain control logic may experience performance limitations due to a lack of adaptability in respect of changing dynamic characteristics.
An objective of this invention therefore is to provide an active vibration control system having the potential for improved performance.